7 a soil properties determination

FCE 311 GEOTECHNICAL ENGINEERING
7. DETERMINATION OF SOIL PROPERTIES
BY LABORATORY TESTING
Department of Civil & Construction Engineering
University of Nairobi

7.1 Determination of Liquid
Limit
a) Casagrande Apparatus
• Apparatus - mechanical device, a cup
mounted on edge pivot - rests on hard
rubber base.
• Cup lifted by 10mm, dropped on the
base.
• The soil is put in cup and levelled off
horizontally.
• Soil is divided by a standard grooving
tool through the pivot of the cup.

Limit - cont’d
• The two halves of the soil flow together
as the cup is repeatedly dropped onto
the base.
• Number of drops at the rate of two
revolutions per second required to close
the groove over a distance of 13mm is
recorded.
• Test repeated over four times and the
water content is determined each time.

Limit - cont’d
• Water content is plotted against log of
blows.
• Best straight line fitting the points drawn.
• Moisture content at 25 blow is the Liquid
Limit of the soil.

Limit - cont’d
• Schematic representation of the
Casagrande Apparatus
21
N=25 blows
Closing distance =
13mm
(Holtz and Kovacs, 1981)
13mm

Limit - cont’d
• A photograph of the Casagrande
Apparatus

Limit - cont’d
• Flow Curve
22
N
w
Das, 1998

Limit – cont’d
b) Cone Penetrometer Appratus
• The cone approximately 35 mm long
and angle of 30º
+ 1º
.
• The mass of the cone together with its
sliding shaft is 80g.
• A metal cup, about 55mm in diameter
and 40mm deep is used to contain the
test sample.

Limit - cont’d
• The Cone Penetrometer Apparatus
24
(Head, 1992)

Limit - cont’d
• Air dry soil sample (about 250g) passing
the 425 micron sieve is mixed with
distilled water.
• The soil paste filled in the metal cup and
the surface struck off level.
• Cone lowered to just touch the surface of
soil and then released for a period of 5
seconds.
• The penetration is recorded.
• The cone is lifted and cleaned.

Limit - cont’d
• The cone is lifted and cleaned.
• Test repeated over four different
moisture contents.
• The moisture contents used in the tests
should be such that the penetrations
obtained lie within a range of 15 to 35
mm.
• Cone penetration is plotted against
moisture content both on normal scales
to give the best fitting straight line.

Limit - cont’d
• The moisture content corresponding to a
cone penetration of 20mm is taken as
the liquid limit of the soil.
• For all practical purposes is the same as
the LL determined by the Casagrande
apparatus.
• The Cone Peretrometer method gives a
more consistent estimate of the LL than
the Casagrande apparatus, with greater
repeatability and less operator
susceptibility.

Limit - cont’d
• Cone penetration against moisture
content
25
Water content w%
Penetrationofcone
(mm)
20 mm
LL

7.2 Determination of
Plastic Limit
• Sample of soil is mixed with distilled
water until it is sufficiently plastic to be
rolled into a ball between palms of
hands.
• A small portion of the ball is then rolled
on a smooth plate into a thread of 3mm
diameter, and the thread is looked for
signs of cracking.
• If no cracks are seen, the thread is
picked up and again rolled into a ball
between palms.

Plastic Limit – cont’d
• The water content is reduced by the heat
of the fingers.
• The ball is then rolled on smooth plate
into a threat of 3mm diameter.
• The steps are repeated until a 3mm
diameter threat first shows signs of
cracking.
• A portion of the threat is taken for water
content determination which gives the
plastic limit.

Plastic Limit – cont’d
• Rolling on a
smooth plate
• Rolling on between
palms of hands
• Soil thread 3mm diameter just crumbles

Shrinkage Limit
• Defined as the maximum water content at
which there is no reduction in volume of soil
mass accompanying reduction in water
content.

7.4 Atterberg Indices
a) Plasticity Index
• Is defined as Liquid Limit minus Plastic Limit
PI = LL – PL

b) Flow Index, IF
• Is the slope of flow curve obtained by plotting
water content as ordinate or natural scale
against number of blows as abscissa on
logarithmic scale






−
=
1
2
21
log
N
N
ww
IF

c) Toughness Index
• Defined as the ratio of Plasticity Index to Flow
Index.
IT =

d) Consistency Index
• Defined as the ratio of Liquid Limit minus
natural Water Content to the Plasticity Index.
IT =
P
C
I
wLL
I
−
=

e) Liquidity Index
• Defined as the ratio of natural Water Content
minus Plastic Limit to Plasticity Index.
IT =
P
L
I
PLw
I
−
=

7.5 Use of Consistency Limits
• Consistency limits and related indices are very
useful for soil identification and classification.
• The limits are often in specification for soil
compaction and in semi-empirical methods of
design.
• The liquid limit and plastic limit depend on both
the type and amount of clay.
IT =

IT =
Plasticity Index Plasticity
0 Non-Plastic
<7 Low Plastic
7 - 17 Medium Plastic
>17 Highly Plastic

• Typical values of Atterberg Limits:
IT =

7.6 Water Content
a) Oven-drying method
• Usual standard laboratory method in
which the soil is dried in an oven at 105º
C
to 110ºC.
• Most accurate method.
• The temperature range of 105 to 110º
C
• Drying up to 24 hours or more depending
upon the amount and type of soil. The
water content is always reported on the
basis of the dry weight of the soil sample.
IT =

7.6 Water Content
b) Sand Bath method
• Field method requiring only about one
hour’s drying period.
• A sand bath (a basin containing about
50mm thick sand layer) is heated over a
gas burner or stove.
• Wet soil sample kept in a container, is
placed on the sand bath and heated until
dry. The sand is stirred with a palette knife
(or a rod) during heating to assist drying.
IT =

7.6 Water Content
c) Other methods
• Infrared lamp and torsion balance method,
calcium carbide method and nuclear
method can be used for rapid
determination of water content.
IT =

7.7 Specific Gravity
• Knowledge of the specific gravity of soils is
required for the determination of voids
ratio, degree of saturation, and in the
sedimentation and consolidation tests.
• The individual mineral particles
constituting a soil have different specific
gravities.
• The specific gravity can be determined
either by the use of a 50 - 100ml density
bottle, a 500ml flask or a pycnometer.
• Density bottle method is the usual
laboratory method.
IT =

7.7 Specific Gravity – cont’d
• Phase diagram for determination of
Specific Gravity
IT =
SIVA
Copyright©2001
7
(i)
Empty bottle
(Mass M1)
(ii)
With dry soil
(Mass M2)
(iii)
With soil and
water (Mass M3)
(iv)
With water
(Mass M4)

• Mass of soil solids
= M2 – M1
• Mass of water in (iii)
= M3 – M2
• Mass of water in (iv)
= M4 – M1
IT =
SIVA
Copyright©2001
7
(i)
Empty bottle
(Mass M1)
(ii)
With dry soil
(Mass M2)
(iii)
With soil and
water (Mass M3)
(iv)
With water
(Mass M4)

• Hence mass of water
equivalent in volume of
that of soil solids
= Mass of water in (iv)
minus
Mass of water in (iii)
= (M4 – M1) – (M3 – M2)
IT =
SIVA
Copyright©2001
7
(i)
Empty bottle
(Mass M1)
(ii)
With dry soil
(Mass M2)
(iii)
With soil and
water (Mass M3)
(iv)
With water
(Mass M4)

IT =
SIVA
Copyright©2001
7
(i)
Empty bottle
(Mass M1)
(ii)
With dry soil
(Mass M2)
(iii)
With soil and
water (Mass M3)
(iv)
With water
(Mass M4)
waterofvolumeequivalentofMass
solidssoilofMass
=G
)()( 2314
12
MMMM
MM
−−−
−
=
)()()( 434312
12
MMM
M
MMMM
MM
d
d
−−
=
−−−
−
=

7.7 Determination of Field
Density
a) Core Cutter method
• A core cutter is a steel
cylinder open at both
ends with one end
sharpened to form the
cutting edge.
• The usual dimensions
are 10cm internal
diameter and height
about 12 to 15 cm.
IT =

Density
• The internal diameter, height and mass of
core-cutter are noted.
• The place where the field density is to be
determined is cleared of shrubs, if any,
levelled and the core-cutter is placed
vertically on the ground surface.
IT =

Density
• A steel ring about 2.5cm height (Steel
Dolly) is placed on top of the core cutter
and is gently driven into the ground by
blows of a rammer, until the top of the
steel ring is nearly flush with the ground
surface.
• Sufficient soil is excavated from around
the core-cutter to enable a person to put
his hands and lift the core-cutter with soil
inside off the ground.
IT =

Density
• The core cutter with soil inside is brought
to the laboratory, the ends are trimmed,
levelled and weighed.
• The soil is removed from the core-cutter
and the samples are taken from top,
middle and bottom positions for water
content determination.
• The average of the three determinations
gives the in-situ water content.
IT =

Density
b) Sand replacement method
• Small pouring cylinder: suitable for fine
and medium grained soils.
• Metal tray to excavate the hole with
suitable shape and size.
• Calibration container of the small
pouring cylinder (size 100 x 150mm).
• Large pouring cylinder: suitable for fine,
medium and coarse grained soils.
• Calibration container
IT =

Density
• Medium pouring cylinder: suitable for line,
medium and coarse grained soils.
• Tools for levelling and excavating.
• Containers.
• Sand.
• Balance
IT =

Density
IT =

Density
• The unit’s base plate is laid on the
compacted surface and material is
excavated through the hole in the plate to
a depth of about 150 mm.
• This wet material is weighed, dried in an
oven and weighed again to determine the
moisture content.
IT =

Density
• The volume of the hole is measured by
filling it with dry, free-flowing sand from a
special sand-cone cylinder.
• Since the density of the sand is known, the
volume of the hole is calculated. The
density (wet unit weight) of the compacted
sample is found by dividing the weight of
the material by the volume of the hole.
IT =

Density
IT =
w
d
+
=
1
ρ
ρ
Dry unit weight is found by using
the formulae;

7 a soil properties determination

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